GERMAN seabird ecologists have found Christmas Island brown boobies to be highly vulnerable to any depletion in prey during breeding season.
Marine immune-ecologist Dr Nina Dehnhard says the male Brown booby (Sula leucogaster), which is physically smaller than the female, has a weaker immune system and suffers a general loss of body condition when its mate is on the nest.
She found the male Red-tailed Tropicbirds (Phaethon rubricauda), who also alternated foraging duties with their mates, to be faring much better during chick rearing season.
“They have a lot of similarities [same breeding island, same chick-rearing time, rear only one chick],” she says.
“But then differ strongly in their foraging range, and in the fact that sexes are dimorphic in brown boobies, but not in Red-tailed Tropicbirds.
“For brown booby males, chick-rearing is more stressful than for the females.
“Likely, the size-dimorphism is related to this, as the smaller size makes it more difficult for the males to forage.
“Studies on other islands with GPS and dive-recorders have shown that males have to fly more actively [more wing-beats] and can dive less deep than the females.
She says Red-tailed Tropicbird males and females are about the same size, and share the foraging load.
“Both sexes do long foraging trips during the incubation period and alternate with incubating the eggs,” she says.
“During chick-rearing, when chicks need food regularly, the parents (both sexes) make alternating short- and long foraging trips.
“The short-foraging trip enables them to feed the chick with a decent meal, during the long foraging trip, the adults can regain energy-reserves for themselves.
“This probably allows them to keep stress levels low and energy reserves for their immune system up.”
Her supervisor, Dr Janos Hennicke, has been making regular research trips to Christmas Island for 10 years in a series of studies called the Seabird Project.
This is an ecological investigation of all Christmas Island seabird species, including their environmental conditions, behaviour, foraging ecology, physiology and immunology.
“Nina’s project was a part of it because we wanted to know how costly reproduction is with respect to the investment into the chick and the investment into the immune system,” Dr Hennicke says.
“Seabirds are long-lived so they have to basically decide the trade-off, because you can only invest into one of those.
Brown boobies are common on the northern coast of Australia and breed on the islands off north-west WA and on the Great Barrier Reef in Queensland.